1. Field of the Invention
This invention relates to a process for bleaching pulp without using organic chlorine compounds but still achieving levels of brightness required by the market e.g. 85-90 ISO. Before bleaching, pulp is continuously digested in a modified continuous cooking (MCC) digester (e.g. that described in U.S. Pat. No. 5,080,755 and U.S. patent application, Ser. No. 08/051,396 each incorporated by reference, herein). Subsequent to MCC digestion, the pulp undergoes oxygen-delignification in a modified continuous (MC) oxygen delignification process (e.g. that described in U.S. Pat. No. 3,963,561, incorporated by reference herein), until a low lignin level is achieved, e.g. low Kappa numbers of 10 or less.
The novel process not only achieves good brightness and low lignin levels without using organic chlorides to bleach, but allows for very low levels of bleaching liquid discharge such that total filtrate, from washing of pulp during the bleaching process, that leaves the bleaching plant may be limited to 3 to 10 tons of liquid per ton of 90% pulp. Essentially, the novel process comprises treating the delignified pulp with sulfuric acid and a chelating agent such as EDTA, washing pulp in a single or double-diffuser washing tower (e.g., that described in U.S. Pat. No. 3,348,390 (single diffuser) and U.S. Pat. Nos. 3,563,891, 4,840,047 and 4,971,694 (double-diffusers), each of the foregoing patents' disclosures are incorporated by reference, herein), bleaching the pulp with hydrogen peroxide in a single or double-diffuser washing tower, adding sulfuric acid and ozone with mixing, reacting ozone with the pulp in a reaction vessel, and washing the ozone reacted pulp in a washing tower, wherein liquid filtrates from the aforesaid washing steps are recirculated to earlier washing steps and where single or double-diffuser washing steps have a high efficiency of at least 85%.
2. Description of the Prior Art
The environmental protection authorities are making ever more stringent demands on the pulp industry to decrease the use of chlorine gas in bleaching. Permitted discharges of organic halogen compounds (AOX) in the waste water from bleaching plants have been gradually lowered and are now at such a low level that the pulp factories have in many cases stopped using chlorine gas. Instead, only chlorine dioxide is used as a bleaching agent. In achieving the same bleaching effect, chlorine dioxide forms smaller quantities of AOX than does chlorine gas. However, the polluting effects of chlorine dioxide have likewise been questioned. On the one hand, the environmental protection authorities in certain countries require that the discharges of organic chlorine compounds be reduced to such a low level that the requirements can hardly be fulfilled even if only chlorine dioxide is used for bleaching. On the other hand, environmental movements in several countries, in particular in Germany, have persuaded consumers to demand paper products which have been bleached entirely without using either chlorine gas or chlorine dioxide.
The pulp industry is therefore searching for methods which permit pulp to be bleached without using these chemicals. One such method has been developed by the Swedish company Eka, which supplies bleaching chemicals to the pulp industry. The bleaching method, which is called LIGNOX (e.g. as described in SE-A-8902058), involves the unbleached pulp being first delignified with oxygen and then, after washing, being treated with EDTA, or other suitable chelating agent, in order to remove heavy metals bound within the pulp. The EDTA treatment stage (denoted as Q state, hereafter) is followed by an intensive bleaching stage with peroxide (denoted as P stage, hereafter), i.e. hydrogen peroxide. The charge of hydrogen peroxide (H.sub.2 O.sub.2) employed is relatively high, 15-35 kg per ton of pulp, depending on the brightness required and on the bleachability of the pulp. The time is quite long, 4 hours or longer, and the temperature high, 80.degree.-90.degree. C.
However, the LGNOX method only provides a limited increase in brightness. Maximum brightness depends on the bleachability of the pulp and the charge of peroxide. Brightness in the region of 80-82 ISO has been achieved using the LIGNOX method. To achieve higher levels of brightness, further bleaching stages are required over and above the peroxide stage.
In this connection, ozone is an interesting bleaching chemical. At least one experiment has shown that if an ozone bleaching stage (denoted Z stage, hereafter) is introduced after a peroxide stage, a significant increase in brightness is achieved while at the same time lignin content of the pulp is decreased. The latter point is important, since a pulp bleached with only peroxide or oxygen/peroxide still contains a relatively high content of lignin, which affects the brightness reversion tendency of the pulp. When high lignin content pulp is warmed or irradiated with sunlight, the pulp yellows. If ozone is used, further lignin is removed, resulting in the brightness of the pulp becoming more stable.
A.G. Lenzing (See EP-A-441 113) has demonstrated how an ozone stage after a peroxide stage increases the brightness of sulphite pulp. If a peroxide stage is allowed to follow the ozone stage, a further increase in brightness is obtained.
Eka has shown that this is also the case for sulphate pulp. Oxygen-bleached sulphate pulp was treated with EDTA to remove heavy metals and subsequently the pulp was bleached with peroxide and ozone according to the stage sequence QPZ. With this sequence, brightness in the region of 82-87 ISO was achieved, depending on the type of pulp. By extending the bleaching sequence with a further peroxide stage and bleaching according to the stage sequence QPZP, brightness in the region of 87-89 ISO was obtained, depending on the type of pulp, See "Non Chlorine Bleaching," J. Basta, L. Andersson, W. Hermanson; Proceedings Mar. 2-5, 1992--Westin Resort--Hilton Head--South Carolina; Copyright by Miller Freeman, Inc.
Thus, it is possible, using process stage sequences QPZ and QPZP, to achieve the levels of brightness which the market requires for paper pulp, i.e., 87 ISO and higher, without using chlorine-containing bleaching agents. This provides interesting perspectives regarding both the effect of cellulose factories on the environment and the possibility of satisfying the demands of consumers for access to chlorine-free bleached pulps.
A prerequisite for achieving high levels of brightness while using moderate quantities of bleaching agents is that, prior to bleaching, the pulp should have been delignified to low kappa numbers, at least lower than kappa number 16.
Normally, a deterioration in quality, in particular loss of fiber strength, is obtained if the delignification in the digester house and oxygen-delignification are taken too far. However, using the modified digestion methods which have been developed in recent years, it has been found possible to achieve very low kappa numbers without loss of strength.
For example, it is possible, using a modification of Kamyr's continuous digestion processor or modified continuous cooking (MCC) as described in U.S. Pat. No. 5,080,755 and U.S. patent application Ser. No. 08/051,396, combined with modified continuous oxygen-delignification, e.g. that described in U.S. Pat. No. 3,963,561, to achieve and go below kappa number 10 with softwood, and kappa number 8 with hardwood, while retaining their strength properties. The modification of the MCC process involves the "Hi-heat" washing zone in the lower part of the continuous digester also being utilized for counter-current digestion (See for example U.S. patent application Ser. No. 07/583,043, incorporated by reference herein). This is achieved by heating to the full digestion temperature in the "Hi-heat" circulation and adding alkaline digestion liquid to this circulation. The total digestion time in countercurrent is thereby extended to 3-4 hours as compared with about 1 hour in conventional MCC. In this way a very low concentration or lignin is achieved by the end of the digestion, which provides improved selectivity in the delignification, i.e. the lignin of the wood is efficiently eliminated without the cellulose being significantly affected. The digestion and oxygen-delignification can thus be carried out to very low kappa numbers without impairing the properties of the pulp.
SUMMARY OF THE INVENTION
The present invention provides a novel process for bleaching pulp without chlorine containing chemicals and for greatly reducing total washing filtrate discharge from the bleaching plant. The process is carried out in a bleaching plant using washing presses and/or single-diffusers (one-stage diffusers) and/or double-diffusers (two-stage diffusers) washers. The novel process comprises the steps of:
1. sending pulp, that has been subjected to continuous digestion and then oxygen-delignification and then washing in a washing apparatus, through a washing press; PA1 2. treating, with agitation, the delignified washed pulp with sulfuric acid and a chelating agent; PA1 3. washing the pulp in a washing apparatus; PA1 4. bleaching with hydrogen peroxide in a reaction vessel; PA1 5. washing the hydrogen peroxide bleached pulp in a washing apparatus; PA1 6. adding sulfuric acid and ozone with mixing of the pulp; PA1 7. reacting the pulp with ozone in a reaction vessel; PA1 8. washing the ozone reacted pulp in a washing apparatus;
wherein the majority of liquid filtrate from the bleaching process that is waste not to be recycled is drawn off from the washing apparatus of step 3,
the washings in step 3 and 5 have at least 85% efficiency,
filtrate from washing step 5 is recirculated and used as washing liquid for the washing apparatus of step 3,
and filtrate from the washing apparatus of step 8 is recirculated and used in the washing apparatus for the washing of pulp after oxygen-delignification.
Alternately, the novel process may use an additional bleaching step 9 of bleaching with hydrogen peroxide using a peroxide charge of e.g. 1-3 kg. per ton of pulp and washing the pulp after it receives a second hydrogen peroxide bleaching, in a washing apparatus, e.g. a washing press, a single-diffuser washer, or double-diffuser washer,
wherein the majority of liquid filtrate from the bleaching process that is waste not to be recycled is drawn off from the washing apparatus of step 3,
the washings in step 3 and 5 have at least 85% efficiency,
the filtrate from washing step 5 is recirculated and used as washing liquid for the washing apparatus of step 3,
the filtrate from the washing apparatus of step 8 is recirculated and used in the washing apparatus for the washing of pulp after oxygen-delignification,
and the filtrate from the step 9 washing is recirculated and used as washing liquid in the ozone washing of step 8.
The washing apparatus used in steps 3, 5, 8, and 9, mentioned above may be selected from the group consisting of a washing press, a single-diffuser washer, or a double-diffuser washer. Preferably a single-diffuser is used most preferably a double-diffuser is used.
A further alternative bleaching step 9 is bleaching the pulp with hydrogen sulphite and washing in a washing apparatus, e.g. a washing press, a single-diffuser washer, or a double-diffuser washer: preferably a single diffuser, most preferably a double-diffuser.